The world depends on hydrocarbons to solve many of its energy needs. Consequently, oilfield operators strive to produce and sell hydrocarbons as efficiently as possible. Much of the easily obtainable oil has already been produced, so new techniques are being developed to extract less accessible hydrocarbons. One such technique is steam-assisted gravity drainage (“SAGD”) as described in U.S. Pat. No. 6,257,334, “Steam-Assisted Gravity Drainage Heavy Oil Recovery Process”. SAGD uses a pair of vertically-spaced, horizontal wells less than about 10 meters apart.
In operation, the upper well is used to inject steam into the formation. The steam heats the heavy oil, thereby increasing its mobility. The warm oil (and condensed steam) drains into the lower well and flows to the surface. A throttling technique is used to keep the lower well fully immersed in liquid, thereby “trapping” the steam in the formation. If the liquid level falls too low, the steam flows directly from the upper well to the lower well, reducing the heating efficiency and inhibiting production of the heavy oil. Such a direct flow (termed a “short circuit”) greatly reduces the pressure gradient that drives fluid into the lower well.
Short circuit vulnerability can be reduced by carefully maintaining the inter-well spacing, i.e., by making the wells as parallel as possible. (Points where the inter-well spacing is smaller than average provide lower resistance to short circuit flows.) In the absence of precision drilling techniques, drillers are forced to employ larger inter-well spacings than would otherwise be desirable, so as to reduce the effects of inter-well spacing variations. Precision placement of neighboring wells is also important in other applications, such as collision avoidance, infill drilling, observation well placement, coal bed methane degasification, and wellbore intersections for well control.
Electromagnetic (EM) ranging solutions have been developed to directly sense and measure the distance between pipes is nearby wells as the drilling commences in the latter well. Some multi-well EM ranging techniques are not cost effective as they involve multiple teams to deploy one or more wireline tools in an existing well, while a logging-while-drilling (LWD) is deployed in the new well being drilled. Meanwhile, an example single-well EM ranging technique (U.S. Pat. No. 7,812,610) relies on absolute magnetic field measurements for distance calculation, which does not produce reliable results due to variations of the current on the target pipe.
Another EM ranging technique, referred to herein as surface excitation ranging, utilizes a current source located at earth's surface and a target well. Specifically, current from the current source is provided to a metal casing of the target well, which causes the target well to emit EM fields along its length. The EM fields emitted from the target well can be used to guide drilling of a new well near the target well. Due to current leakage from the target well into the surrounding formation, surface excitation ranging can produce weak EM fields and poor signal-to-noise ratio (SNR) for sensors in deep wells. Increasing the amount of current injected into the target well would improve the EM field strength and SNR available for ranging, but such increases in current are not always possible for a given power supply and can be a safety hazard to workers at earth's surface.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.